Low-grade insults meant to undermine the self-confidence of a woman so she might be more vulnerable to your advances. This is something no decent guy would do. They say that the assholes get the girls, but I can spot negging a mile away and I reject these fuckers straight off.

Everywhere there is an insecure pretty girl, there is some guy negging.
Negging can be so subtle, it's pratically undetectable.
I was wondering why that guy was complimenting me while putting me down. He was negging of course.

The practice of methodically killing other players in a massively multiplayer game, who have dramatically less power than oneself. A term originated in games in which players spawn with a negative bounty score, to deter such strategies. This term originated in the game, Sub Space, from Virgin Interactive. Rather than deter the practice, negative bounties only served to increase it, due to the novelty of a large negative score. Subsequently, negative bounties were eliminated. The practice if killing extremely low bounty players, as a strategy and pass time, continued with the same name.

It's a way to pick up girls. How it works is you use remarks to tap into female insecurity; Shake their confidence.
Neg is a negative remark wrapped in a back-handed compliment.
So your neg will confuse and intrigue them and maybe even shake their confidence a little bit, but only enough for them to fall from the clouds and be interested in talking to you. Its way to get threw their defenses at bars and such.

An example of Negging a girl would be:
"You are nearly as tall as me. I like tall girls (LIFT). Are those heels 4 or 5 inches (DROP)?"
OR
You say, "Ahhh, that's so funny ... you nose moves when you speak...... (pointing and being cute) look there it goes again ... its so... quaint ... haha look"
She'll say, "ahhh, stoppp!"*blush*.
Now she is self conscious and having her in this state is where you want her. You have neghit her.